Refactor Praefect metadata
This MR has several smallish refactorings with regards to Praefect metadata:
- it unifies the naming of
FooFromContext
- functions to serialize and deserialize transactions are now private
- Praefect server metadata is now serialized and deserialized via two common functions instead of being implemented twice, respectively
- naming of metadata and environment variables was aligned with other internals, mostly by adding the "gitaly-"/"GITALY_" prefix